Various known apparatuses especially spiral freezers which use a liquid cryogen, such as carbon dioxide or nitrogen, are essentially operated as "isothermal" freezers, i.e. operating at one low temperature throughout the cooling or freezing chamber. As a result, the exiting cryogen gas is close to the cold temperature of the device, and some of the cooling capabilities of the gas and efficiency are lost. The cooling apparatus described in U.S. Pat. No. 4,356,707 is an example of an isothermal freezer using carbon dioxide.
In respect to most food products, a liquid cryogen coolant, such as nitrogen, has the ability to provide cooling at a faster rate than food products can thermally accept. Most food products, for example, accept freezing rapidly at the beginning of the freezing process, and thereafter the ability of the food to thermally accept freezing slows. This can lead to some inefficiencies. Moreover, some loss of cryogen may occur through the entrance of the cooling device as products, such as food products, enter it.
When cooling apparatuses use an endless conveyor belt having a spiral or helical section to convey food into and through a refrigeration zone, the belt, which generally is porous, generally collapses along its inner edge as it begins to turn from its straight entrance section into its spiral or helical conformation as it moves through the cooling apparatus. Because of this collapsing, food or other frangible items carried by the conveyor belt can be harmed as a result of a pinching action exerted by the collapsing belt on food that may be frozen thereto when the belt and the food turns into the helical section. If food should be frozen to the upper surface of the belt, such a pinching action of the belt section which collapses may often result in tearing and damage to the food surface. Further, cooling apparatuses which use a cryogen and spiral conveyor belts have had the proclivity to allow the cryogen gas to exhaust through the intake or discharge opening for the belt. Generally the intake opening is at a lower part of the apparatus; and the discharge opening for the belt usually is at the upper portion of the apparatus, the belt taking a load vertically up through the cooling chamber for cooling. In such cooling apparatuses, such flow of the cold vapor from a cryogen exiting the inlet opening is not controlled so as to exact maximum BTUs for cooling.
It is an object of this invention to provide a cabinet cooler or freezer which uses a liquid cryogen for refrigeration and extracts maximum benefit from such liquid cryogen to rapidly and efficiently lower the temperature of a product load.
It is another object of this invention to provide a cabinet freezer which uses a liquid cryogen and which freezer includes means for contacting a product load with the liquid cryogen as the load enters the freezer to initially withdraw heat from the load, and to freeze the surface of the products so as to stop evaporative dehydration, thus minimizing shrink.
Another object of the invention is to provide a liquid cryogen cooler or freezer with an exhaust which generally provides circulation of vapor from the liquid cryogen such that a temperature gradient is created within the freezer with the exhaust exiting at relatively warm temperatures.
Still another object of the invention is to provide a freezer which controls cryogen vapor exiting from an inlet to the freezer so that at least about 80% of the cryogen vapor of the total vapor is circulated through a temperature gradient within the freezer and exits from the outlet of the freezer at relatively warm temperatures.
Yet another object of the invention is to provide a method for providing cooling to a product load in a freezer such that the cooling of the freezer matches or correlates with the ability of the product load to accept freezing.
These and other objects of the invention should be apparent from the following detailed description for carrying out the invention when read in conjunction with the accompanying drawings.